Biosensors are known as sensors that detect specific components in liquid samples such as body fluids.
Biosensors refer to sensors that utilize biological materials, such as microorganisms, enzymes, antibodies, DNA, and RNA, as molecule identification elements. That is, a biosensor quantifies or detects a specific component contained in a liquid sample by utilizing a reaction that occurs when a biological material recognizes the specific component, for example, oxygen consumption by respiration of microorganisms, enzyme reaction, light emission, etc.
Among various biosensors, enzyme sensors have been extensively commercialized. For example, enzyme sensors which are sensors for glucose, lactic acid, cholesterol, amino acids and the like are utilized in medical measurements, the food industry, etc.
Such an enzyme sensor quantifies a substrate contained in a liquid sample, for example, by reducing an electron mediator by electrons that are produced by a reaction between a specific substance contained in the liquid sample and e.g., an enzyme, and electrochemically measuring the amount of the reduced electron mediator with a measuring device.
In using such a sensor that measures a specific component in a liquid sample, it may become necessary to remove substances that are essentially unnecessary for reaction, substances that can adversely affect reaction and the like from the liquid sample. For example, when a cholesterol sensor contains a surfactant in its reaction system, it may become necessary to remove hemocytes from blood which is a liquid sample. This is because hemocytes are destroyed by the surfactant so that reducing substances contained in the hemocytes, such as glutathione, may adversely affect reaction.
Specifically, it is common to employ a structure of providing a filtration part for removing hemocytes near an opening to which a liquid sample is supplied. As the structure of the filtration part, for example, three systems as illustrated in FIGS. 6 to 8 have been proposed. Therein, filtration is done with a filter provided in the filtration part.
FIG. 6 is a schematic view showing a horizontal separation system. With the system as shown in this figure, blood which is a liquid sample is dropped to the vicinity of the end (primary end) of a filter on the side where a liquid sample is supplied, and the blood is filtered in the horizontal direction. Blood plasma is caused to ooze from the end (secondary end) of the filter on the side where a filtered liquid sample oozes out (see International Publication No. WO 02/054054 A1, for example).
Also, FIG. 7 is a schematic view showing a vertical separation system. With the system as shown in this figure, blood is directly dropped to the primary end of a filter and is filtered in the vertical direction, and blood plasma is caused to ooze from the bottom face of the filter or the vicinity thereof at the secondary end.
FIG. 8 is a schematic view showing a combined separation system. With the system as shown in this figure, blood is directly dropped to the primary end of a filter and is filtered in the vertical direction and then the horizontal direction, and blood plasma is caused to ooze from the secondary end of the filter (see International Publication No. WO 02/095385 A1, for example). With any of these systems, the use of a preferable filter enables removal of hemocytes before they reach the reaction system.
However, the sensors employing these systems of FIGS. 6 to 8 involve directly dropping a liquid sample to the primary end of the filter or the vicinity thereof, and such structure causes the following problems.
First, it is difficult to adequately regulate the amount of a liquid sample to be measured and supply it to the filtration part. Thus, the amount of a liquid sample supplied to the filtration part tends to become excessive or insufficient. Second, it is difficult to continuously supply a liquid sample to the filter at an appropriate speed. Hence, it is possible that a liquid sample is not supplied at a speed commensurate with the filtration performance of the filtration part. As a result of these first and second problems, the function of the filtration part is not fully exhibited, thereby causing a decrease in measurement accuracy, a decline in within-run reproducibility, and an increase in measurement time.
Third, when dropping a liquid sample, the user needs to keep the sensor substantially horizontally. Also, when the user drops blood from his/her finger, the user must drop blood accurately to a proper drop position. That is, further improvements are necessary also in terms of user operability.
For example, International Publication No. WO 03/074999 A1 proposes a biosensor equipped with a liquid sample supply inlet in the shape of a trapezoid that is like a reverse cone; however, this proposal also cannot solve the above-mentioned third problem. Further, when the amount of a liquid sample dropped is small, there is also a problem in that this liquid sample adheres to the portion in the shape of a trapezoid like a reverse cone, not duly reaching the filtration part.
It is therefore an object of the present invention to provide a sensor that is capable of supplying a liquid sample to a filter promptly and readily even if it is not held substantially horizontally, and that is capable of supplying even a small amount of a liquid sample to the central part of the filter promptly and readily.